I have a Canon 8600 flatbed scanner that works well, a friend had a bunch of 8X10 glass plates that scanned suprisingly well for the condition they are in. Two "buts"... if an area of photo is blown out it is too black for my scanner to process, also an area of deep shadow is too white to get any detail. If the exposure is "OK" I get a good negitive file that I can work with.
The over/under exposed plates are something I have been working on to try to light them from below and take a picture of the plate, several actually, and stack them in PS. So far, no workable process. Also, I'm only playing around with about 50 total, several hundred sounds overwhelming. I would be intrested to know how it works out for you and if you find a service that does it.Good luck.

I've been scanning some collodion wet plates that are in some cases tin, others glass. Our Epson V750 with Silverfast 8 is as good as anything I know of. Our plates are not as large as yours, so you'll probably run into some difficulty trans-illuminating a plate as large as 9x12 on anything that's a consumer scanner.

I have gotten excellent results with an Epson V700 scanner. You will need to cut a custom carrier out of black matte board, but it is very easy to do.

It might be hard and expensive to find a service to help you. Since you have several hundred, I bet you could buy an Epson V700 for less than the service would charge. I would only use a local service where I could hand deliver the fragile plates.

As mentioned, the dynamic range of some plates can exceed the range of a single pass. There are two solutions. Silverfast 8 Plus will do multiple scans and combine. I have also done multiple scans manually and used HDR software to combine the images.